A surge arrester is a device for connection between electrical terminals in order to prevent (arrest) an increase in voltage across those terminals if such voltage were to exceed a certain predetermined device dependent value. Surge arresters are thus utilized to protect personnel and equipment from undersirable, and usually unpredictable, momentary surges of electrical potential. Such devices exhibit very high impedance between terminals in order not to interfere with the normal functioning of the protected equipment until a voltage surge exceeding their threshold appears. They then switch (breakdown) to a low impedance, surge arresting mode, in order to dissipate the surge power by conducting it away from the protected terminals. When the surge ceases an arrester returns to its normal, high impedance mode.
An important parameter of surge arresters is the relative dependence of their actual breakdown voltage on the swiftness with which the surge voltage rises. The design value of breakdown voltage is based upon a slow rate of voltage rise, i.e., on the order of 100 volts per second. It is known that the breakdown voltage value increases with increasing surge voltage rates of rise. This is because of an inherent response time of the device, and surge arresters exhibit different actual breakdown thresholds with different surges.
This circumstance is expressed in the art by dividing the actual breakdown voltage with a fast rising surge by the design breakdown voltage (i.e., that for slow surges), which quotient is termed "impulse ratio". Thus an ideal surge arrester would have an impulse ratio of 1:1 for all surges fast or slow, because it has only one value of breakdown voltage.
A gas-filled surge arrester with improved impulse ratios is disclosed in U.S. Pat. No. 4,084,208, issued Apr. 11, 1978 to Bazarian and Bonneson. The therein disclosed arresters are said to have an impulse ratio of less than 2:1.